As wind turbines are built for constantly greater power, transmission elements transmitting the torque between the rotor of the wind turbine and a power machine must be dimensioned up correspondingly.
According to the prior art, a generator is often positioned in an elevated nacelle at the rotor. The torque is transmitted from the rotor to the power machine, which is generally constituted by an electric generator, via a gearbox. The gearbox is necessary for stepping up the relatively low rotational speed of the rotor to a rotational speed which is suitable for the generator.
The total weight of the gearbox and generator is considerable. Especially for offshore wind turbines such big masses positioned at a relatively high level above the seabed mean that the supporting structures will be comprehensive and expensive.
It is known to transmit the torque from the rotor via transmission elements to a generator which is at ground level, see for example German patent application 2932293.
The transmission of the torque via a simple, vertical shaft at substantially the same rotational speed as the rotor is conditional on the supporting structure of the wind turbine being dimensioned for absorbing the full torque about its vertical axis. This condition is met by using two concentric counter-rotating shafts for the torque transmission. Thereby the torque is substantially equalized, whereby the supporting structure is not subjected to said torques. U.S. Pat. No. 4,311,435 deals with a typical device for torque transmission of this kind.
The torque of the shafts is relatively great when rotational speeds are used which correspond to the rotational speed the rotor. Shafts and other transmission elements for the purpose may therefore be both big and expensive.